It is commonly known that differential thermal expansions between components of a reactor have to be taken into account in order to avoid damage that may occur when the temperature of the reactor is changed. A common practice is to divide each channel connecting such components in separate upstream and downstream parts to allow relative movement of the components due to differential thermal expansion. In order to maintain the channel sealed, the upstream and downstream parts are connected by an air-tight flexible means, such as fabric or metal bellows. An expansion joint with fabric bellows is commonly used when relatively large movements or movements both in the axial and the lateral directions are expected.
High temperature expansion joints that are capable of handling relatively large thermal movements are often required in channels connected to the particle separator of a circulating fluidized bed (CFB) boiler. A problem is that fabric bellows cannot withstand the high temperatures of the gas and particles flowing through the particle separator. Moreover, particles packing between the moving parts of the joint may prevent the movement, and thereby cause breakage of the joint.
U.S. Pat. No. 5,366,255 discloses a high temperature expansion joint arranged between two portions of a channel. In the area of the joint, one of the channel portions telescopically overlaps the other channel portion in a spaced relationship to permit relative axial and lateral movements of the channel portions. Each channel portion comprises an outwardly extending extension, which outwardly extending extensions are sealingly connected with a composite fabric belt dimensioned to permit the relative movements. Thermal insulation is disposed between the outwardly extending extensions to inhibit heat flow to the fabric belt. A drawback in the joint is that the thermal insulation may be irreversibly tightened in repeated compressions, so that particulate material is allowed to pack into the joint, and damage the fabric belt, or prevent the movements of the joint.
Finnish Patent No. 87271 discloses a high temperature expansion joint between an upstream portion and a downstream portion of a channel, which are sealingly connected by a flexible element connected on the outer side of the channel. The joint comprises intermediate elements that are connected in the downstream channel portion in a way that allows axial, but not lateral movement, of the intermediate elements. The intermediate elements are supported by a spring against a transverse plane of the upstream channel portion and comprise a downstream end arranged to slide on an inner circumference of the downstream channel portion. A drawback of this expansion joint is that at least when the system is used at a temperature lower than its highest operating temperature, the inner circumference of the downstream channel portion, on which the intermediate element fits in a high temperature operation, is directly accessible to abrasive forces due to particles flowing in the channel. Therefore, the construction has a risk of wearing of the sliding surface on the inner circumference of the downstream channel portion in start-up and close-down phases of a fluidized bed boiler. Moreover, the maintenance of the expansion joint is complicated, inasmuch as it can only be done inside the channel.
Finnish Utility Model No. 20110203, now Finnish Patent Document No. FI 9485, discloses a high temperature expansion joint that differs from that in Finnish Patent No. 87271, mainly in that axial extensions of the intermediate elements, which elements are movably connected to a first channel portion, are arranged to slide on an outer circumference of the first channel portion. Moreover, the intermediate elements are in the case of a vertical channel supported against a transverse end plane of the second channel portion by gravitation only. A drawback of this expansion joint is that at least when the system is used at a temperature lower than its highest operating temperature, the sliding surface of the axial extensions of the intermediate elements, on which the first channel portion fits in high temperature operation, is directly accessible to abrasive forces due to particles flowing in the channel. Therefore, the construction has a risk of wearing of the sliding surface on the axial extension of the intermediate elements in start-up and close-down phases of a fluidized bed boiler.